A model for orbital pacing of methane hydrate destabilization during the Palaeogene

Abstract

A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eocene, about 59 to 50 million years ago. The events, although variable in magnitude, were apparently paced by orbital cycles 2-4 and linked to massive perturbations of the global carbon cycle 5,6. However, a causal link between orbital changes in insolation and the carbon cycle has yet to be established for this time period. Here we present a series of coupled climate model simulations that demonstrate that orbitally induced changes in ocean circulation and intermediate water temperature can trigger the destabilization of methane hydrates. We then use a simple threshold model to show that progressive global warming over millions of years, in combination with the increasing tendency of the ocean to remain in a more stagnant state, can explain the decreasing magnitude and increasing frequency of hyperthermal events throughout the early Eocene. Our work shows that nonlinear interactions between climate and the carbon cycle can modulate the effect of orbital variations, in this case producing transient global warming events with varying timing and magnitude. From the late Palaeocene to the early Eocene (∼59-50 Myr), Earth's surface and oceans went through an interval of progressive warming, culminating in the early Eocene climatic optimum (EECO, ∼51 Myr; ref. 6; Fig. 1). Superimposed on this gradual warming trend are a series of 'hyperthermal' events-geologically abrupt (<10 kyr) warmings of Earth's surface and deep ocean, the most prominent of which being the Palaeocene-Eocene Thermal Maximum (PETM, ∼56 Myr; ref. 6). Two subsequent smaller events, ETM2 (∼54 Myr; ref. 2), and ETM3 (∼53 Myr; ref. 7) seem to share similar characteristics. Associated with the hyperthermals are large negative carbon-isotope excursions of surficial carbon reservoirs and dissolution of deep-sea carbonate (Fig. 1; refs. 8), consistent with massive injections of 13 Cdepleted carbon into the ocean-atmosphere system. Proposed sources for this carbon include methane hydrates 5 , terrestrial peat deposits 9 , and thermogenic methane 10. Understanding the causes of hyperthermals is important, not least because the amount of the carbon release and magnitude of global warming can be used to estimate climate sensitivity. The hyperthermals also reflect possible threshold events ('tipping points'), with slow changes in boundary conditions giving rise to rapid positive feedback and state transitions in the carbon-climate system 11-13. Cyclostratigraphic evidence from complete marine sections suggest that the PETM, ETM2, and ETM3 events all initiated on maxima in the 100 kyr eccentricity cycle 2-4. This has led to the suggestion that orbital pacing controlled the timing of carbon injection 2,4 and hence the occurrence of the hyperthermal events.

Reconciliation of marine and terrestrial carbon isotope excursions based on changing atmospheric CO2 levels

“…Although our reconciliation does not allow for the determination of the carbon source of the CIE at the PETM, recent work indicates significant warming before the onset of the CIE (refs 33,48), and suggests an orbitally forced mechanism for the release of carbon at the event 49 (1) and (2) simultaneously.…”

Frequency, magnitude and character of hyperthermal events at the onset of the Early Eocene Climatic Optimum

“…It has been theorized that the timing and magnitude of the hyperthermals would respond to the crossing of a thermal threshold, more frequently reached in phases of orbital-driven temperature increase Lunt et al, 2011). In addition, the carbon reservoir or capacitor (Dickens, 2003), regardless of its nature and as a result of the long-term temperature increase from the late Paleocene to the early Eocene, would be largely depleted by the peak of the EECO, leading to an interval free of hyperthermals.…”

The 405 kyr and 2.4 Myr eccentricity components in Cenozoic carbon isotope records

“…They may play a role in the Oligocene and Miocene, however (Pälike et al, 2006;Holbourn et al, 2015;Tian 5 et al, 2013), but could also be related to changes in ice-volume (see below). Given the intermediate ocean depth temperature response to orbital variability and seasonal contrast in a fully coupled climate model (Lunt et al, 2011), methane hydrate dynamics could be a plausible explanation for coupled δ…”